1. Field of the Invention
The present invention relates to electronic components and methods for making the same. In particular, the present invention relates to a monolithic ceramic electronic component having a ceramic element including ceramic layers and internal electrode layers and to a method for making the same.
2. Description of the Related Art
Dielectric ceramic materials having perovskite structures, such as barium titanate, strontium titanate and calcium titanate, have been widely used in capacitors due to the high specific dielectric constants thereof. Trends towards miniaturization of electronic components require more compact capacitors having large electrostatic capacitance.
Since conventional monolithic ceramic capacitors using dielectric ceramic materials as dielectric layers are sintered at temperatures as high as approximately 1,300.degree. C., noble metals such as palladium must be used as internal electrode materials. The use of such expensive noble metals inevitably increases the material cost of the capacitors.
The use of base metals in internal electrodes of monolithic ceramic capacitors is progressing for solving the above problem, and various dielectric materials having reduction resistance and capable of sintering in neutral and reducing atmospheres have been developed to prevent oxidation of electrodes during sintering.
A further reduction in size and a further increase in capacitance are required for monolithic ceramic capacitors, and technologies are being developed for achieving higher dielectric constants of dielectric ceramic materials, thinner dielectric ceramic layers and thinner internal electrode layers.
When the thickness of the ceramic layer disposed between the internal electrode layers is reduced to 3 .mu.m or less, unevenness of the interface between the dielectric ceramic layer and the internal electrode layer increases or defects or pores in the dielectric ceramic increase, resulting in shorter service lives.
A reduction in particle size of the powdered ceramic material is proposed in order to improve smoothness of green ceramic sheets for forming ceramic layers and to increase the density of the green ceramic sheet (Japanese Unexamined Patent Application Publication No. 10-223469). As the particle size decreases, the powdered ceramic readily agglomerates, resulting in poor dispersibility. Thus, the improvement in the surface smoothness and the density of the green ceramic sheet is not sufficient only by the reduction in particle size. Moreover, the dielectric constant of the powdered ceramic decreases as the particle size decreases in the same composition, and the reduction in particle size is not suitable for monolithic ceramic capacitors having higher capacitance.
As the size of the metal particles used in internal electrodes decreases, the initial sintering temperature of the metal particles decreases, and delamination will readily occur. It is difficult to use such metal particles as electrode materials for monolithic capacitors.
When the content of organic binders in ceramic is increased in order to improve the surface smoothness of a green ceramic sheet, the volume fraction of the powdered ceramic in the green ceramic sheet is decreased and the volumetric shrinkage of the ceramic element (chip) increases during sintering. When the volumetric shrinkage of the ceramic element is large, the area of the electrode paste on the green ceramic sheet also decreases in response to the areal shrinkage of the green ceramic sheet. Since the volume of the electrode material, such as nickel, is constant in the internal electrode, the thickness of the internal electrode layer unintentionally increases contrary to the trends toward thinner multilayers.
In a green ceramic sheet containing a large amount of organic binder and having a large areal shrinkage, the thickness of the electrode paste applied thereon can be reduced in consideration of the areal shrinkage of the green ceramic sheet. The reduction in thickness, however, results in the formation of pinholes in the electrode paste layer and an increase in surface roughness of the electrode due to a decreased leveling of the electrode paste. These defects decrease the electrode coverage (effective electrode area) after sintering, resulting in deterioration of electrical characteristics of the product.
The above-described problems occur also in various monolithic ceramic electronic components other than the monolithic ceramic capacitors.